When Daisy says this line, she has just returned to the dinner table after a brief return to the house because her husband, Tom, had received a phone call. While Daisy is inside, Jordan reveals that the caller is Tom's mistress in New York who frequently telephones the house, much to Daisy's anger and dismay.
That Daisy speaks this line with "gaiety" suggests that she is keen to maintain appearances. She wants to appear as her usual happy and cheerful self and to avoid arousing any suspicion among her guests. That this gaiety is "tense," however, implies that Daisy is struggling to maintain this facade and that deep down, she is very unhappy in her marriage. Similarly, this "tense" tone of voice also displays her embarrassment: she knows that her guests are aware of Tom's affair but she does not want to admit it.